Communication systems are evolving into advanced systems that provide mobile stations with services capable of high-speed, high-capacity data transmission and reception. An Institute of Electrical and Electronics Engineers (IEEE) 802.16e communication system is a typical example of such a communication system. In the IEEE 802.16e communication system, communication between a mobile station (MS) and a base station (BS) is constantly kept in an operation of a normal mode.
In the IEEE 802.16e communication system, the mobile station normally monitors the downlink to receive data from the base station. The mobile station usually monitors the downlink even when the base station has no data to transmit to the mobile station and/or the mobile station has no data to transmit to the base station, thus causing continual power consumption in the mobile station.
Because the IEEE 802.16e communication system considers mobility of the mobile station, the power consumption of the mobile station is an important factor in the entire system performance. Therefore, to minimize the power consumption of the mobile station, a sleep mode operation between the mobile station and the base station, and an awake mode operation corresponding to the sleep mode operation have been proposed. In addition, to cope with the change in the channel state with the base station, the mobile station periodically performs a ranging operation for correcting timing offset, frequency offset, and power with the base station.
FIG. 1 illustrates a sleep mode operation in a general communication system.
Referring to FIG. 1, in step 101, a mobile station (MS) 100, if it desires to transition from an awake mode to a sleep mode, transmits a MOBile-SLeeP-REQuest (MOB-SLP-REQ) message to a base station (BS) 110. Upon receipt of the MOB-SLP-REQ message, the BS 110 determines in step 103 whether to grant the mode transition to the sleep mode of the MS 100 taking the conditions of the BS 110 and MS 100 into account, and transmits a MOBile-SLeeP-ReSPonse (MOB-SLP-RSP) message to the MS 100 according to the determination result. The MOB-SLP-RSP message includes a listening interval parameter. In the listening interval, the BS 110, if it has data to transmit to the MS 100, can transmit a MOBile-TRaFfic-INDication (MOB-TRF-IND) message including an identifier (ID), or indicator, of the MS 100.
Upon receipt of the MOB-SLP-RSP message from the BS 110, the MS 100 starts the sleep mode operation in accordance with the MOB-SLP-RSP message. The MS 100 recognizes that it should perform the operation in accordance with the listening interval parameter included in the MOB-SLP-RSP message. In addition, the MS 100, although it is in the sleep mode state, can immediately operate in the awake mode in the sleep mode state when it has data to transmit to the BS 110.
In step 105, the BS 110 transmits a MOB-TRF-IND message to the MS 100 in the listening interval of the sleep mode. Here, the MOB-TRF-IND message is assumed to be a message not including the identifier of the mobile station (MS). Because the MOB-TRF-IND message is a message not corresponding to the MS 100, the MS 100, after decoding the MOB-TRF-IND message, performs the sleep mode operation again, determining that its own identifier is not included therein.
After a lapse of a predetermined time, if the BS 110 has data to transmit to the MS 100 in the listening interval of the sleep mode, it transmits a MOB-TRF-IND message including an identifier of the MS 100 in step 107. Because the MOB-TRF-IND message is a message corresponding to the MS 100, the MS 100, after decoding the MOB-TRF-IND message, transitions to the awake mode and receives data from the BS 110, determining that its own identifier is included therein.
After the data transmission/reception between the MS 100 and the BS 110 is completed, the MS 100 and the BS 110 exchange again a MOB-SLP-REQ message and a MOB-SLP-RSP message to make mode transition back to the sleep mode. By exchanging the MOB-SLP-REQ message and the MOB-SLP-RSP message for the mode transition to the sleep mode, the MS 100 and the BS 110 may perform unnecessary message transmission, thereby wasting the uplink and downlink resources and causing the power consumption. In addition, the MS 100 should transmit a BandWidth-REQuest (BW-REQ) message to receive a bandwidth allocated for transmitting the MOB-SLP-REQ message to the BS 110 and should perform bandwidth ranging, thereby causing a delay in the time for which the MS 100 makes the mode transition to the sleep mode.